Along with development of highly computerized society, many investigations for applying optical technology to transmission, processing and recording of information have been conducted, in order to efficiently treat a large amount of information. In this context, liquid crystalline compounds are expected to be significantly useful material, which can control properties of light such as wavelength, transmittance and polarization in an adjustable manner.
It is known that liquid crystalline compounds are utilized in optical materials such as nonlinear optical materials and photochromic materials. Displays on which characters and images are displayed and optical compensator are known as manufactured products made with the optical material. On the other hand, the liquid crystalline compounds are known as raw material of fiber or reinforced plastics for example, in addition to the optical material. Further, the liquid crystalline compounds are expected to be applied to tribology material, electrically conductive material, semiconductive material, and luminescent material in which extraordinary properties of the liquid crystalline compounds are utilized.
Liquid crystalline compounds are generally classified into two categories. One of the categories is calamitic liquid crystal composed of rod-like molecules, and the other is discotic liquid crystal composed of disc-like molecules. As an example of practical application, in existing liquid crystalline displays, the rod-like liquid crystalline molecules are used in the part for performing optical switching, and the discotic liquid crystalline compound are used in an optical compensation film (a phase difference plate).
Discotic (disc-like) liquid crystalline compounds having a broad π-conjugated plane as a core forms a columnar (column-like) phase in which the disc-like compounds are stacked to each other in a self-organizing manner. In the columnar phase, the π-conjugated planes are accumulated to the center of the column in one dimensional manner, to form one-dimensional channel. This one-dimensional channel provides high charge transporting properties, which are comparable to those of an organic single crystal and amorphous silicon. Besides, the discotic liquid crystalline compounds have been extensively investigated over the world, as a fundamental material for next-generation printable electronics. This is because superior properties of the discotic liquid crystalline compounds such as self-orienting properties, solubility and flexibility are significantly advantageous for device production in a simple solution process.
Discotic liquid crystalline compounds are in disc-like shapes and have a central core (cyclic core) and radially extending side-arms attached to the core. Orientation of the disc-like compounds causes optical anisotropy (polarizability), electronic anisotropy (electron or charge transporting properties), and mechanical anisotropy (viscosity). By making full use of such properties, it is expected to utilize the discotic liquid crystalline compound as display material, optoelectronic material, photonic material and tribologic material. Further, negative optical anisotropy, which is the characteristic property of the disc-like molecules, is most generally utilized in an optical compensation film for liquid crystalline display, at the present time. Thus, there is a great need in the present industrial community to develop novel discotic liquid crystal having novel optical properties and/or processability.
The above-describe various properties are caused by the orientation state of the liquid crystalline molecules. Thus, it becomes possible to optically switch the charge transporting properties of the liquid crystalline molecules, if the orientation state of the liquid crystalline molecules can be controlled by phase transition caused by a stimulus of light in an adjustable manner. Such light stimulus-responsive liquid crystalline material is useful as information recording material in optical memories, for example. However, phase transition of discotic liquid crystalline phase caused by light has not been reported until now. Shimizu et al reported a discotic liquid crystalline compound in which photosensitive azobenzenes are attached to the disc-like compound (triphenylene) as the side arms (see NPL 1). However, the structural change of the whole molecule caused by isomerization of azobenzene moieties is not large enough to induce the phase transition, because the photo-responsive moieties are introduced in the side arms.
Further, organic compounds having a photo-induced phase transition property between a crystalline (solid) phase and an isotropic phase have not been found yet.